Fulvalene structure and dipolar character. X-ray crystal structures of 1-benzyl-2-cyclopentadienylidene-1,2-dihydropyridine, 1-(2',6'-dichlorobenzyl)-4-cyclopentadienylidene-1,4-dihydropyridine, and 2-(2',6'-dichlorobenzyl)-1-cyclopentadienylidene-1,2-dihydroisoquinoline